PURPOSE: To effectively reduce electric power losses without impairing the comfort of air conditioning in a room.

CONSTITUTION: An inverter frequency decision device 16 decides the inverter frequency f based on inputs of room temperature Ta and set value Ts. When a low frequency region judging device 18 judges that f belongs to the low frequency region, it sends the judged signal to a V/f controller 17. The V/f controller 17 increases the inverter electric power temporarily only when it judges whether or not a lock arises at a compressor 1 but almost always operates an inverter section 13 with an inverter output voltage V at which the inverter input electric current I becomes minimum.